1. Field of the Invention
The present invention relates to a gas separator for a fuel cell, and a fuel cell using the same gas separator for a fuel cell, and more particularly relates, in a fuel cell composed by laminating plural unit cells, to a separator for a fuel cell provided between adjacent unit cells for forming together with adjacent members a fuel gas passage and an oxidizing gas passage, and for separating fuel gas and oxidizing gas, and a fuel cell using such a separator.
2. Description of the Related Art
A gas separator for a fuel cell is a member for composing a fuel cell stack of laminated plural unit cells, and has a gas impermeability sufficient to prevent mixture of fuel gas and oxidizing gas supplied to adjacent unit cells. Such separator for a fuel cell usually has a surface which is ribbed or formed in another corrugated sectional structure, and which forms passages for fuel gas and oxidizing gas (gas separators having such structure are also called ribbed interconnectors). Such a separator for a fuel cell, when incorporated into a fuel cell stack, forms a passage for fuel gas or oxidizing gas (unit cell inside passage) between an adjacent member (gas diffusion layer) and this corrugated structure.
This gas separator for a fuel cell also has a specified hole aside from the corrugated structure for forming the gas passage. When this fuel cell stack is formed by laminating unit cells having gas separators, holes corresponding to adjacent gas separators overlap with each other to form a gas manifold penetrating the fuel cell stack in its laminating direction. Such a gas manifold is used for passing the fuel gas or oxidizing gas from the outside of the fuel cell to the inside thereof and for distributing the fuel and oxidizing gas into individual unit cells as well as for leading the fuel exhaust gas or oxide exhaust gas discharged after electrochemical reaction in each unit cell to the outside of the fuel cell. Therefore, the manifolds communicate with the unit cell inside passages formed in the laminated unit cells, so that the gas flows in and out between the gas manifolds and the unit cell inside passages.
FIG. 32 is a plan view explaining the structure of a separator 130 exemplary of gas separators of the related art. The separator 130 has four holes in four sides, that is, holes 140, 143 for air, and holes 150, 152 for fuel gas. These air holes 140, 143 and fuel holes 150, 152 form, when assembled into the fuel cell by laminating members including the separator 130, an oxidizing gas feed manifold, an oxidizing gas exhaust manifold, a fuel gas feed manifold, and a fuel gas exhaust manifold, respectively in the fuel cell.
At one surface of the separator 130, a rib 155 is formed for communicating between the air hole 140 and the air hole 143, and at the other surface of the separator 130 (back surface of the drawing), there is a rib (not shown) for communicating between the fuel hole 150 and the fuel hole 152. Herein, these ribs are groove structures formed in parallel. When composing the fuel cell by laminating the members including the separator 130, these ribs form unit cell inside gas passages between the members adjacent to the separator 130. That is, the rib 155 for communicating between the air hole 140 and the air hole 143 forms a unit cell inside oxidizing gas passage, and the rib communicating between the fuel hole 150 and the fuel hole 152 forms a unit cell inside fuel gas passage. The oxidizing gas supplied into the fuel cell passes through the oxidizing gas feed manifold formed by the air hole 140, is distributed into the unit cell inside oxidizing gas passages formed in the individual unit cells, is collected in the oxidizing gas exhaust manifold after electrochemical reaction, and is discharged to the outside of the fuel cell. Similarly, the fuel gas supplied into the fuel cell passes through the fuel gas feed manifold formed by the fuel hole 150, is distributed into the unit cell inside fuel gas passages formed in the individual unit cells, is collected in the fuel gas exhaust manifold after electrochemical reaction, and is discharged outside of the fuel cell.
In such a fuel cell for obtaining an electromotive force by presenting fuel gas and oxidizing gas for electrochemical reaction, it is desired to enhance the utility rate of the supplied gas. That is, in the fuel cell, gas (fuel gas or oxidizing gas) containing electrode active material (hydrogen or oxygen) is supplied, but all of the electrode active material in the gas is not utilized in electrochemical reaction, and in order to promote the electrochemical reaction sufficiently, gas containing the electrode active material exceeding the theoretically required amount is supplied into the fuel cell. It is therefore desired to increase the gas utility rate to suppress the amount of gas supplied into the fuel cell by presenting the electrode active material in the gas so as to be utilized sufficiently in the electrochemical reaction. Moreover, the oxidizing gas is desired to be capable of suppressing the amount of energy consumed in pressurizing the oxidizing gas (usually air), and capable of enhancing the energy efficiency of an entire system having such a fuel cell.
To enhance the gas utility rate by making the electrode active material in the gas more easily utilized in the electrochemical reaction, it is required that the gas be agitated and diffused sufficiently in the passage. As a result, contact between the catalyst provided in the electrode and the electrode active material is improved. In order to agitate the gas and diffuse it sufficiently in the passage, for example, it is known to increase the flow rate of gas passing through the passage in the unit cell inside passage, to accelerate the flow velocity. To realize such a method, the sectional area of the passage of the unit cell inside passage may be decreased. As such, a serpentine structure has been proposed for the shape of the corrugated structure that defines the unit cell inside passages formed on the gas separator (for example, Japanese Laid-open Patent No. 7-263003). Herein, the gas to be supplied into each unit cell is introduced into the fine passage formed continuously on the same plane. Therefore, if the volume of gas supplied from the outside into the fuel cell is equivalent, as compared with the structure shown in FIG. 32 in which the gas is passed simultaneously into a wider range on the same plane in each unit cell, the flow velocity of the gas passing through an arbitrary position in the passage is faster, so that the gas utility range is enhanced.
However, if the corrugated structure formed on the gas separator is of such serpentine structure, since the unit cell inside passage is folded into small pieces on the same plane, a pressure loss increases when the gas passes through the passage. Therefore, in order to maintain the flow rate of the gas passing through the passage at a specified rate, it is necessary to further pressurize the gas to be supplied into the fuel cell, and the energy consumed in pressurizing the gas increases, thereby lowering the energy efficiency of the entire system having this fuel cell.
Aside from the above related art, it has also been proposed to divide the gas passage formed on the separator into plural regions (for example, Japanese Utility Model No. 58-138268). In such a fuel cell, the gas passage divided into plural regions is formed on a gas separator (bipolar plate). The gas supplied from the gas feed hole into the unit cell passes sequentially through the plural regions and is discharged from the gas discharge port. In such a constitution, too, the flow velocity of the gas passing through the passage is accelerated, and the gas utility rate is enhanced. However, like the serpentine structure, since the gas flow is continuous in the unit cell, and the gas collecting holes are mutually connected through a diaphragm, it is difficult to solve the problem of pressure loss sufficiently. In the constitution of this related art, moreover, since the gas flow is continuous in the unit cell, the gas may not be distributed uniformly into each unit cell sufficiently.
Besides, when decreasing the sectional area of the unit cell inside passage as mentioned above, the corrugated structure formed on the gas separator must be much finer. That is, it is required to manufacture the gas separator at high precision. When manufacturing the gas separator, however, it is difficult to enhance the precision when forming the corrugated structure on the surface. If the precision is insufficient, it may lead to lowered manufacturing yield (increase of defectives), or fluctuations of cell performance due to deteriorated precision in forming the corrugated structure.